Historically, all commercially produced motorcycles have been equipped with wire spoke wheels which are relatively light in weight and which have adequate strength and resilience in normal operation of the motorcycle.
Wire spoke wheels, however, have certain disadvantages such as the necessity of using a tire inner tube, the necessity of periodic tightening of the spokes to avoid unsafe riding and handling conditions of the vehicle, the abrasion of the tire inner tube, and the danger of puncturing the tire inner tube by the head of a broken spoke.
Moreover, wire wheels have an inherent weakness when subject to severe impacts, such as when crossing chuckholes or bumps. Although wire spokes are effective against tensile forces absorbed in the side of the wheel away from the impact, they do not function against compression forces and thus do not provide radial support of the rim in resisting deformation at the point along the rim perimeter subjected to severe impact. In leaning a motorcycle on curves at high speeds, the rims of wire spoke wheels are especially stressed and tend to twist relative to the neutral axis of the rim. Hence, the warping of rims, the fracturing of rims along circumferential break lines, the stretching and loosening of spokes that are commonplace under severe service conditions are important as a basis for improvements in the manufacturing of motorcycle wheels.
In departing from wire wheel construction for motorcycles, lightweight construction must be maintained through the use of lightweight metals such as aluminum, magnesium, or alloys thereof. In any attempt to construct motorcycle wheels from these metals, this invention is based on the premise that it is most practical from a commercial standpoint to make the wheels as castings. It is known that there have been attempts to manufacture motorcycle wheels in the form of light metal castings. These efforts have been generally unsuccessful, particularly in building wheels for the medium to large size motorcycles. An insurmountable difficulty in forming wheels as a single casting, probably attributable to the wheel configuration itself, is the tendency to have porous rims if the hubs are good and, if the casting practice is changed to get good rims, then the hubs are porous.
Porous metal is weak and prone to cracking or yielding in the high stress areas of the rim or in those portions of the hub which support bearing races, etc. Porous metal is further unacceptable in the rims of wheels used with tubeless tires since the sealing compounds normally used on rim surfaces do not seal this type of porosity. Moreover, porosity is unacceptable in any portion of the wheel which must be threaded to receive cap screws or other threaded elements.
Yet another difficulty encountered with wheels formed of a single casting is the inevitable presence of shrink fractures which are believed due to shrinkage of the hub portion to a greater extent than the rim portion.
Nonetheless, there is great inducement to equip motorcycles with wheels formed from lightweight metal castings because there is a great need to reduce the unsprung weight of motorcycles. For example, a cast aluminum alloy wheel according to the invention can be substantially lighter than a wire wheel of the same tire size and permits the use of a tubeless tire. The elimination of the inner tube results in a further ten to fifteen percent reduction of total wheel weight.
Another problem that arises in the casting of motorcycle wheels is the complicated web structure of many rear wheel hubs. Such hubs, if formed of a lightweight metal, must frequently contain steel or iron inserts which provide such elements as brake drum surfaces and bearing race supports. Most rear wheel hubs define recesses for rubber drive compensators. Some hubs have bosses to which brake discs may be applied.
Hence, objects of the invention are: to provide lightweight wheels for motorcycles which are superior in strength, road life and freedom from maintenance, labor, and cost when compared with conventional wire spoke wheels.
A further object is to provide motorcycle wheels of a design enabling the casting thereof by technologically modern methods, such as those employing the use of permanent molds in the casting of such lightweight metals as aluminum, magnesium, and alloys thereof, and to use casting technology which will permit the use of weldable lightweight metals and alloys in casting the wheels.
It is a further object to adopt a method of casting motorcycle wheels which avoids porous metal formation in the castings.
An object also is to provide a wheel unit having a central open region of standard dimension suitable for receiving a variety of hub assemblies insertable thereunto which enables use of a standard wheel unit with hub assemblies of standard exterior configuration but which may vary as to brake design, power coupling and the like.
Another object ancillary to the foregoing object is to achieve a wheel design which substantially reduces the cost of molds through the use of a standard wheel unit with a variety of hub units thereby obviating the necessity for providing an equal variety of complete wheel molds.
An object ancillary to the foregoing object is to devise a wheel uniquely adapted for mass production assembly methods enabling assembly of two basic interchangeable parts.
A further object is to design a motorcycle wheel which achieves the foregoing functional objects with such visual features as to be commercialy acceptable to the trade.